1）Activity1跳转到Activity2的生命周期流程

 1.Activity1启动：Activity1: onCreate()Activity1: onStart()Activity1: onResume()2.点击按钮跳转到Activity2:Activity1: onPause()Activity2: onCreate()Activity2: onStart()Activity2: onResume()Activity1: onStop()3.从Activity2中返回：Activity2: onPause()Activity1: onRestart()Activity1: onStart()Activity1: onResume()Activity2: onStop()Activity2: onDestroy()4.Activity1退出Activity1: onPause()Activity1: onStop()Activity1: onDestroy()

(2) 假设Activity2为一个透明的activity
activity跳转的生命的周期如下

 1.Activity1启动：Activity1: onCreate()Activity1: onStart()Activity1: onResume()2.点击按钮跳转到Activity2:Activity1: onPause()Activity2: onCreate()Activity2: onStart()Activity2: onResume()3.从Activity2中返回：Activity2: onPause()Activity1: onResume()Activity2: onStop()Activity2: onDestroy()4.Activity1退出Activity1: onPause()Activity1: onStop()Activity1: onDestroy()

(3) 那在Activity之上弹出一个系统对话框，生命周期变化呢

 1.Activity启动：Activity1: onCreate()Activity1: onStart()Activity1: onResume()2.点击按钮弹出系统对话框这里发现并没有任何生命周期变化 ，原本还以为会走 onPause() 这个方法。